1. Field of the Invention
The present invention relates generally to voltage controlled oscillators (VCOs) and more particularly to a VCO using a single transistor Colpitts oscillator circuit and having a negative feedback network configured to provide ultra-low phase noise.
2. Description of the Prior Art
A VCO is an electronic circuit with an oscillator having a frequency control input whereby a control voltage can vary the frequency of oscillation. Numerous types of oscillators can be used in VCOs, each with specific advantages and constraints. VCOs often serve as building blocks for more complex circuits, including phase-locked loops and voltage-to-frequency converters. VCOs and circuits incorporating them are used in cellphones and other radio communication devices, and with ever-increasing demands for wireless device performance are subject to ever-increasing requirements for performance and efficiency.
The oscillator circuit in a simple VCO generally is a harmonic oscillator consisting of an amplifier that provides adequate gain and a resonant circuit that feeds back signal to the input. Often the resonant circuit is an LC tank circuit having in its simplest form a capacitor and inductor. Oscillation occurs at the resonant frequency where a positive gain arises around the loop. A commonly used type of oscillator is the Colpitts oscillator, wherein two capacitors and a single inductor determine the frequency of oscillation around a single transistor. The positive feedback needed for oscillation is taken from a voltage divider made from the two capacitors. A well-known method for using a voltage to control the oscillation frequency is by using a voltage-controlled capacitor, also known as a varactor. Any reverse-biased semiconductor diode has some amount of voltage-dependent capacitance and can be used to change the frequency of an oscillator by varying a control voltage applied to the diode. Special-purpose variable capacitance varactor diodes are available with well-characterized wide-ranging values of capacitance and are frequently used in the fabrication of VCOs.
As with any electronic device, a VCO has inherent noise sources. A VCO noise source of major concern in radio applications is phase noise, which is also described as short-term frequency stability. Phase noise is undesirable because the phase noise components of a signal at a desired frequency spread the power of the signal to adjacent frequencies, resulting in noise sidebands and reducing power at the desired frequency. Phase noise in VCOs can be reduced by careful selection of active components and by various circuit design strategies, generally including the addition of a negative feedback loop from the oscillator output back to the oscillator input.
U.S. Pat. No. 5,714,914 entitled HIGH POWER LOW-NOISE VOLTAGE-CONTROLLED OSCILLATOR teaches a VCO comprising a resonant circuit, an active circuit including a transistor connected in a Colpitts configuration, and a buffer amplifier. The oscillation frequency control voltage is applied directly to the cathode of a single varactor diode. The noise reduction strategy for this single-transistor oscillator includes having capacitances connected in parallel with the junction capacitances of the transistor, each capacitance being greater than the respective junction capacitance and serving to suppress feedback at low frequencies. A resistor is also provided to supply negative feedback and suppress gain and noise at low frequencies. In this circuit, low frequencies are defined as frequencies far below the VCO's oscillation frequency. However, the disadvantage of this VCO is that the additional capacitances thereof limit the application at high frequencies as the capacitances may result in the cessation of oscillation.
U.S. Pat. No. 6,075,421 entitled VOLTAGE CONTROLLED OSCILLATOR teaches a Hartley configuration VCO comprising a voltage controllable variable resonant circuit, an amplifier, a positive feedback path and a frequency dependent negative feedback path. The frequency control voltage is applied through an inductor to the common cathodes of a pair of varactor diodes. This VCO is taught with two parallel coupled transistors but the general topology would be equally valid for a single transistor or for more than two transistors coupled in parallel. For purposes of simplicity, the parallel-connected nodes of the transistors will be referred to in the singular. The emitter node of the transistors is the positive feedback node and the collector node of the transistors is the negative feedback node. The frequency dependent negative feedback path consists of a capacitor and inductor in series between the collector and the base of the transistors, with a DC biasing resistor connected intermediate the feedback capacitor and feedback inductor.
Disadvantages of this VCO include the level of phase noise reduction provided, disclosed to be less than −130 dBc/Hz at any frequency shown. Additionally, use at high frequencies may be limited because when more transistors are required more pads are required with the concomitant result of increasing parasitic effects. Further, the negative feedback circuit is combined with the DC biasing circuit, thus constraining both the biasing and negative feedback topologies.
U.S. Pat. No. 6,353,368 entitled VCO CIRCUIT USING NEGATIVE FEEDBACK TO REDUCE PHASE NOISE teaches a CMOS based VCO circuit including a ring oscillator composed of linked inverters, wherein the number of inverters has a primary effect on determining the oscillation frequency. A control voltage applied to each inverter provides a degree of control over the oscillation frequency. The ring oscillator output is fed into a frequency to voltage converter having a voltage output which is fed into a bias circuit, where the bias circuit output voltage is in turn coupled to the control voltage input for the ring oscillator, thereby providing negative feedback to said oscillator circuit. This negative feedback in turn reduces the phase noise in the VCO output signal.
This VCO topology requires the use of potentially numerous CMOS inverters to provide a specific frequency, and the addition of a frequency to voltage converter and bias circuit to form the negative feedback loop. Additionally, no quantitative information is disclosed as to the actual degree of phase noise reduction provided by this circuit, thus leaving the actual degree of benefit unknown. Further, the component matching requirements of this circuit certainly would require it to be implemented only as an integrated circuit, rather than having the option of implementing it with discrete components.
U.S. Pat. No. 7,414,488 entitled LOW PHASE NOISE DIFFERENTIAL LC TANK VCO WITH CURRENT NEGATIVE FEEDBACK teaches a differential mode oscillator circuit with current mirror controlled biasing circuitry and an LC tank circuit connected to a cross-coupled pair of transistors to induce positive feedback and thus oscillation. A pair of varactors with a common anode connection are used to control the oscillation frequency. A third and fourth transistor are used, one with each of the transistors in the cross-coupled pair, to implement a negative feedback loop for each of the oscillator transistors. The phase noise reduction of this circuit is only −110 dBc/Hz for the frequencies shown. Additionally, the component matching requirements of this circuit certainly would require it to be implemented only as an integrated circuit, rather than having the option of implementing it with discrete components.
A higher-performing VCO circuit would have phase noise reduction greater than −160 dBc/Hz and be suitable for fabrication in a wide variety of technologies, including discrete components, hybrid modules and integrated circuits. Further, such a higher-performing VCO would have a simple design and small component count for more economical fabrication.